The present embodiments relate to radio frequency (RF) pulse alignment.
In a magnetic resonance system or magnetic resonance tomography system, the body that is to be examined may be exposed to a relatively high basic magnetic field (e.g., the “B0 field”), of 3 or 7 tesla, for example, with the aid of a basic field magnet system. In addition, a magnetic field gradient is applied with the aid of a gradient system. Radiofrequency excitation signals (RF signals) are then transmitted via a radiofrequency transmit system by suitable antenna devices with the aim of tipping the nuclear spins of certain atoms that have been excited into resonance by the radiofrequency field (e.g., the “B1 field”) in a spatially resolved manner through a defined flip angle with respect to the magnetic field lines of the basic magnetic field. The radiofrequency excitation or the resulting flip angle distribution is also referred to hereinbelow as nuclear magnetization, or “magnetization” for short. The correlation between the magnetization m and the B1 field radiated over a time period T is yielded according to
                    m        ≈                  2          ⁢                      πγ            ·                                          ∫                                  t                  =                  0                                T                            ⁢                                                                    B                    1                                    ⁡                                      (                    t                    )                                                  ⁢                dt                                                                        (        1        )            where γ is the gyromagnetic moment, t the time variable, and B1(t) is the time-variable magnetic field strength of the B1 field. Upon relaxation of the nuclear spins, radiofrequency signals, called magnetic resonance signals, are emitted, received by suitable receive antennas and then processed further. The desired image data may be reconstructed from the thus acquired raw data. The radiofrequency signals intended to produce the nuclear spin magnetization may be transmitted by a “whole-body coil”, also called a “bodycoil”, or sometimes also by local coils attached to the patient or examination subject. A typical structure of a whole-body coil is a cage-like antenna (e.g., birdcage antenna) including a number of transmit rods running parallel to the longitudinal axis that are arranged around a patient chamber of the tomography system in which a patient is disposed during the examination. At the end faces, the antenna rods are in each case capacitively interconnected in an annular arrangement.
It was standard practice in the prior art to operate whole-body antennas in a circularly polarized (CP) mode. A single temporal RF signal is applied to all the components of the transmit antenna (e.g., to all of the transmit rods of a birdcage antenna). The pulses may be transferred with identical amplitude to the individual components offset in phase by a shift aligned to the geometry of the transmit coil. In a birdcage antenna having 16 rods, for example, the rods may be actuated by the same RF magnitude signal, offset in each case with a 22.5° phase shift. The result is then a circularly polarized radiofrequency field extending in the x/y plane (e.g., perpendicularly to the longitudinal axis of the birdcage antenna extending in the z-direction).
In the interim, the radiofrequency signal that is to be transmitted (e.g., the incoming sequence of radiofrequency pulses (referred to as the “reference pulse train”)) may be modified individually in amplitude and phase in each case using a complex transmit scaling factor. The B1 field at a location r (e.g., at a pixel or voxel position r, where r is, for example, a vector having the values of the Cartesian coordinates x, y, z in mm), is given therein according to
                                          B            1                    ⁡                      (            t            )                          =                              ∑                          c              =              1                        N                    ⁢                                                    E                c                            ⁡                              (                r                )                                      ·                                          b                c                            ⁡                              (                t                )                                                                        (        2        )            where bc(t) is an RF curve that is to be transmitted on the channel c=1, . . . , N (e.g., the voltage amplitude characteristic (in V) of an RF pulse train over time t), which is given by bc(t)=SFc·bR(t), where SFc is the complex scaling factor for the channel c and bR(t) is the voltage characteristic of the reference pulse train. Ec(r) is the sensitivity (in μT/V) of the antenna element of the radiofrequency transmit channel c at a specific location r (e.g., the pixel or voxel position). In this case, Ec(r) is the position-dependent sensitivity distribution in the form of a sensitivity matrix.
In this case too, the antenna may be operated in the “CP mode” by selecting the amplitude at the same level for all transmit channels and only providing a phase shift aligned to the geometry of the transmit coil. Depending on the examination object, an elliptically polarized (EP) mode, in which the radiofrequency field in the x/y plane is elliptically rather than circularly polarized, may also be used. Which mode is used may be dependent on the shape of the body region that is to be excited. In the case of objects that are more cylindrically symmetrical (e.g., in the case of images acquired in the head region), the CP mode is more often selected, whereas the EP mode tends to be chosen in the case of more elliptical shapes (e.g., in the case of examinations in the thoracic or abdominal region). The purpose of the EP mode is to compensate for inhomogenities of the B1 field that are caused by the non-circularly symmetrical body shape. In many cases, a technique known as “B1 shimming” of such a multichannel radiofrequency transmit system is performed. In this case, the individual transmit scaling factors are calculated based on a patient-specific alignment generally with the aim of achieving a particularly homogeneous excitation compared to the prior art standard CP or EP mode.
In this case, the transmit scaling factors are calculated using optimizers that minimize the magnitude deviation of the desired perfectly homogeneous target magnetization m from the theoretically attained actual magnetization A·b:b=argbmin(∥A·b−m∥2)  (3)where A is the so-called design matrix, consisting of a system composed of linearly complex equations into which, inter alia, the spatial transmit profiles of the individual transmit channels (e.g., antenna rods) and the present B0 field distribution are inserted. The design matrix is described, for example, in W. Grissom et al., “Spatial Domain Method for the Design of RF Pulses in Multicoil Parallel Excitation,” Mag. Res. Med. 56, 620-629, 2006. In this case, b(t) is the vector of the RF curves bc(t) that are to be transmitted in parallel. If the solution to Equation (3) (e.g., the minimum of the “target function” defined in Equation (3)) is found, the desired scaling factors SF1, SF2, . . . , SFN are available as the result.
For the purpose of establishing the design matrix, appropriate test measurements are first to be carried out in order to determine the coefficients of the design matrix experimentally. The patient to be examined is to remain in the MRT system for the entire duration of the acquisition and calculation of the data for the design matrix as well as of the optimal values for the vector of the RF curves or for b(t). Thereafter, the patient is also to endure the actual examination in addition, which is very stressful, for example, for sick and enfeebled persons. It is not possible to exit the machine in the interim, since otherwise, the entire alignment process, the B1 shimming, is to be repeated due to the unavoidable change in position of the patient.